How to change a *VectorProperty size?
Have you seen Property Definitions? IntVectorProperty have other parameters to define, among which no "size"!

You can create a Custom property from PropertyGroup:

class CustomProperty(bpy.types.PropertyGroup):
    name: bpy.props.StringProperty(name="Name", default="Some name")
    description: bpy.props.StringProperty(name="Description", default="Frequencies")
    size: bpy.props.IntProperty(name="Size", default=2)


To update any property just assign some value:

bpy.types.Scene.a = 3